Significant growth has lately been found in the market of cellular phone systems, typically, such as a Global System for Mobile Communication (GSM) and a Personal Communication Network (PCN) and this tendency is anticipated to continue in the future. One of the requirements of such systems as GSM and PCN is that the output power of portable terminal equipment can be controlled, dependent on the distance from a base station to the equipment. This can be fulfilled by controlling the gain of the power amplifier module installed on the equipment.
FIG. 9 shows an example of a typical conventionally used power amplifier module with three stages of output power control. In this power amplifier module, a signal input through a pin 062 is amplified by first-stage, second-stage, third-stage amplifiers 601, 602, 603, and output through a pin 064. Power source voltage is applied to a pin 063. At this time, an output power control circuit 607 controls the gains of the amplifiers 601, 602, and 603 by changing an idling current that determines a DC bias of transistors 604, 605, and 606. Hetero-Bipolar Transistors (GaAsHBTs) are used as the transistors 604, 605, and 606.
Using the above third-stage amplifier 603 and its output power control circuit 607, the output power control function will be explained below. The amplifier 603 comprises a transistor 606, a resistor 611, coupling capacitance 612, and an output adjustment circuit 613. The output power control circuit 607 comprises transistors 608, 609, and 610 and resistors 614, 615, and 616. Here, the diode-connected transistors 609 and 610 and the diode-connected transistors 608 and 606 form a current mirror circuit. Current that is as large as mirror ratio times the current flowing across the transistors 609 and 610 flows through the transistor 606 as the idling current.
The voltage across the transistors 609 and 610 becomes substantially constant when an output power control voltage applied to a pin 061 becomes higher than the boot voltage of these transistors. In the voltage region higher than the boot voltage, the idling current increases or decreases in proportion to the control voltage. Because the gain depends on this idling current, the gain can be made variable by controlling the idling current. In fact, the output power control uses this characteristic. In the conventional module example shown in FIG. 9, the idling current to flow in the first-stage amplifier 601 is generated by applying a voltage produced by dividing the control voltage by resistance to the base of the amplifier. This means taken is different from the means of idling current supply for the second-stage amplifier 602 and the third-stage amplifier 603.